i) Field of the Invention
This invention relates to a semiconductor device of very excellent reliability and a method for producing the same, and more particularly to a semiconductor device free of any trouble due to alpha-rays from a package and a method for producing the same.
ii) Brief Description of the Prior Art
In general, semiconductor devices are sealed usually by the ceramic package (including also a methodxe2x80x94cerdipxe2x80x94in which the ceramic package is sealed with glass), the plastic package, or the like. Especially in the ceramic package among these packages, ceramic materials contain uranium, thorium etc. on the order of several ppm. As stated in, for example, the xe2x80x9c16th Proceedings of Reliability Physics (1978)xe2x80x9d, at page 33, it has been known that these impurities emit alpha-rays and therefore cause failures (called xe2x80x9csoft errorsxe2x80x9d) in memory devices etc. For this reason, the reliability of the semiconductor devices may lower conspicuously.
On the other hand, when powder, such as alumina, called filler is used in plastic package materials, the impurities are also contained in the filler. The filler is less influential on the integrated circuit devices than in the case of the ceramic encapsulation because it is surrounded with plastic materials such as epoxy resin and the alpha-rays emitted from the impurities are absorbed by the plastics. These plastic materials, however, have the disadvantages of comparatively low reliability in moisture resistance and heat resistance on account of the fact that the moisture absorbability is high, the fact that the contents of alkali impurities typified by Na are high and the fact that the heat resistance is as low as at most about 150xc2x0 C. It is therefore difficult to employ plastic-encapsulated integrated circuit devices in systems of high reliability. Accordingly, integrated circuit devices for use in the high-reliability systems need to be subjected to the ceramic type encapsulation.
For these reasons it has been strongly desired to prevent soft error of the semiconductor memory device caused by alpha-particles coming from uranium and thorium contained in the ceramic package as impurities.
In Electronics, Jun. 8, 1978, pages 42-43, it is shown that this problem is attacking from several different directions, for instance, by devising new packaging material process or applying protective coating to the upper surface of the chip.
However, this article does not disclose the most favorable materials or necessary characteristics of the protective coating in order to prevent soft error caused by alpha particles.
IEEE Journal of Solid State Circuits, vol. SC-13, No. 4, August, 1978, pages 462-467 shows planar multilevel interconnection technology employing a polyimide resin.
However, in this article, the polyimide films having a thickness of 2.5 xcexcm are employed for interlevel dielectrics and final passivation. It must be recognized, however, that such thin films of polyimide cannot prevent penetration of alpha particles coming from outside of the film such as the ceramic package.
The use of the polyimide film as the protective film or coating to prevent soft error of the semiconductor memory device caused by alpha particles is not disclosed in this article.
Also, some prior art references show the use of polyimide resin in the field of the semiconductor devices, however, none of these references show the use of polyimide as a protective film or coating to prevent soft error of a semiconductor memory device caused by alpha particles.
For instance, U.S. Pat. No. 4,017,886 provides a polyimide layer between an SiO2 layer and a metal layer to bond a wire with the electrode very easily by forming a flat upper surface on which the metal layer is formed.
Furthermore, Japanese Patent publications No. 47-12609 and No. 52-26989 show the uses of polyimide for insulation and final passivation, respectively.
There is no disclosure in those prior art references concerning prevention of soft error caused by alpha particles.
This invention has for its object to eliminate the disadvantages of the prior art, and to provide a semiconductor device which maintains a high reliability for moisture and heat exhibited by the ceramic encapsulation wherein the failure of an integrated circuit due to alpha-rays as previously stated, is prevented from occurring; as well as a method for producing such a semiconductor device.
In order to accomplish the object, a semiconductor device according to this invention has a coating film on at least a region of an element in a semiconductor substrate having at least one element, the coating film being made of a polyimide resin or a polyimide isoindoloquinazolinedione resin (hereinbelow, written xe2x80x9cPII resinxe2x80x9d) and being 10 xcexcm or more thick, and it is encapsulated in a ceramic package.
The semiconductor device of this invention causes the polyimide resin or the PII resin to attenuate and absorb alpha particles which emit from impurities contained in a package material. Accordingly, the resin coating film to serve as an attenuating material and an absorbing material is required to be a film thick to the extent of preventing the alpha particles from penetrating therethrough. In order to avoid any fluctuation in the characteristics of the element, the thickness should preferably be at least 10 xcexcm and more preferably be at least 30 xcexcm. The capability of preventing the penetration of the alpha particles is not limited to the resin coating films, but it is generally possessed by insulating films. It is extremely difficult, however, that insulating films of silicon dioxide, phophosilicate glass, silicon nitride, aluminum oxide etc. having heretofore been employed in semiconductor devices are deposited on semiconductor substrates 10 xcexcm or more. More specifically, these insulating films formed by the chemical vapor deposition undergo very high stresses and cause cracks when deposited several xcexcm or more. With the sputtering process, the insulating films can be deposited under the condition under which the proportion of development of the cracks is held comparatively low. However, the deposition rate is as very low as several hundreds xc3x85/min, and it is actually impossible to deposit the films 10 xcexcm or more. In contrast, with the polyimide resin or the PII resin, the stress of the film is as very low as about 4 Kg f/mm2. In addition, the breaking distortion is about 30%, which is approximately one order greater as compared with those of the aforecited inorganic insulating films. Therefore, a thick film of several tens xcexcm can be formed under the condition under which quite no crack develops. On the other hand, among high polymer resins, some possess film forming characteristics similar to those of the polyimide resin and the PII resin. Since, however, the sealing step of the ceramic package is ordinarily executed at high temperatures of around 450xc2x0 C., a heat-resisting property enough to endure the temperatures is required, and no resin other than the aforecited ones satisfies this property.
More specifically, as exemplified in FIG. 1, when various high polymer resins are subjected to thermogravimetric analyses, decreases in weight begin at 200xc2x0-250xc2x0 C. in case of a silicone resin 13 and in case of an epoxy resin 14, whereas a decrease starts at 500xc2x0 C. in case of the polyimide resin 12. In case of the PII resin 11, the heat resistance is more excellent, and the weight residue at 600xc2x0 C. is approximately 70% which is the most excellent. In this manner, the polyimide resin or the PII resin has the heat-resisting property against the high temperature step described previously.
In the PII resin or the polyimide resin, the contents of impurities such as uranium and thorium functioning as alpha-ray generating sources are as very low as 0.1xe2x80x94several ppb or so (the impurity analyses resorted to radioactivation analyses). Accordingly, the PII resin or the polyimide resin stops the alpha-rays emitted from the ceramic package material, and simultaneously, the alpha-rays to be generated by the resin itself become an extremely small amount. On the other hand, it can be generally said that organic high polymer materials are lower in the impurity contents than inorganic materials. However, in case of a polyethylene resin taken as an example of the organic high polymer material, the uranium content is 40-50 ppb which is comparatively high, and the organic high polymer materials are not always suitable.
The coating of the polyimide resin or the PII resin attenuates and absorbs alpha particles coming from uranium and thorium contained in the ceramic package, thereby, penetration of alpha particles and soft error of the semiconductor memory device are effectively prevented. Furthermore, it is necessary that the amounts of alpha particles emitted from the coating film itself is extremely low.
The protective coating film to prevent soft error caused by alpha particles must satisfy following conditions.
(1) The amounts of uranium and thorium contained in the coating film itself is extremely low, i.e. less than 40 ppb, so that the coating film does not emit alpha rays enough to cause soft error of the semiconductor memory device.
(2) The coating film must have a thickness enough to attenuate and absorb alpha rays and must never have cracks.
(3) The coating film should be heat resistant so as to withstand heating in the packaging step (the assembly process includes a step of heating at 300xc2x0 to 500xc2x0 C.).
However, a material which can satisfy these conditions was not known before the present inventors.
The present inventors have found out that only the PII resin and the polyimide resin can satisfy these conditions from experiments requiring a great deal of expense and labor.
That is, it has been widely believed that organic resin materials surely contain remarkable amounts of impurities, such as uranium and thorium, because representative refining means, such as recrystallization or zone-refining are never employed to make the organic resin material.
However, the present inventors have carried out studies of the polyimide resin and the PII resin and have made clear by huge amounts of data that among many kinds of organic resin material, only the PII resin and the polyimide resin can satisfy above conditions but other resin materials can not satisfy them.
It is needless to say that in order to know which organic resin material can satisfy above condition (1), it is necessary to know the amounts of uranium and thorium contained in each resin, respectively.
However, trace analysis of uranium and thorium called for a great deal of expense and labor. The most reliable method of trace analysis of uranium and thorium is the radioactivation analysis by thermal neutrons in the nuclear reactor and no other methods are available that provided reliable data.
This method involves placing the sample of the organic resin material in the nuclear reactor, radiating thermal neutrons to activate the sample and measuring the uranium and thorium contents from the attenuation curve of the gamma-rays generated upon disintegration of the radioactivated element.
This method requires the use of the nuclear reactor for one full day and attenuation of the gamma-rays must be continuously measured for about two days for the test of one sample. Therefore, to obtain the data of five samples of Table 1 4,000,000 yen and a great deal of effort was necessary.
The amounts of uranium and thorium contained in five kinds of organic resin materials measured by the present inventors are shown in Table 1.
From Table 1, it is evident that among five kinds of organic resin material, only the PII resin and polyimide resin can satisfy above condition (1). However, the other resin materials contain more than 40 ppb of uranium and thorium and cannot satisfy condition (1).
Also, it should be recognized that the protective film to prevent the soft error of the semiconductor memory device must have a thickness sufficient to prevent the penetration of alpha particles therethrough coming from the ceramic package.
The capability of preventing the penetration of alpha particles is not limited to the resin coating film. However, the present inventors found out from their experiments, that it is extremely difficult to form an inorganic material film having such a thickness, 10 xcexcm or more.
That is, according to the present inventors"" experiments, it was extremely difficult to form protective films having a thickness of 10 xcexcm or more, of silicon dioxide, phosphosilicate glass, silicon nitride, aluminum oxide, etc., on a semiconductor substrate. More specifically, these inorganic insulating films formed by the chemical vapor deposition undergo very high stresses and cause cracks when deposited at thicknesses of several xcexcm or more. With the sputtering process, the insulating films can be deposited so that development of the cracks is suppressed and the crack development is comparatively low. However, the deposition rate in this process is very low, i.e. on the order of several hundreds xc3x85/min., and it is actually impossible to form a film having a thickness of 10 xcexcm or more.
Accordingly, it is not possible to satisfy condition (2) using an inorganic insulating material.
In contrast with the polyimide resin or the PII resin of this invention, the stress of the film is very low as about 4 kgf/mm2. In addition, the breaking distortion is about 30%, which is one order greater as compared with those of aforesaid inorganic insulating films. Therefore, a thick film of more than 10 xcexcm can be easily formed under the condition under which no crack develops.
On the other hand, among high molecular organic resins, some possess film forming characteristics similar to those of the PII resin and the polyimide resin. Since, however, the sealing step of the ceramic package is ordinarily executed at high temperatures of 300xc2x0-500xc2x0 C., a heat-resisting property enough to endure the temperatures is required, and no resin other than the PII resin and the polyimide resin satisfies this property.
More specifically, as exemplified in FIG. 1, when various organic resin materials are subjected to thermogravimetric analysis, decreases in weight begin at 200xc2x0-250xc2x0 C. in the case of a silicon resin 13 and in the case of an epoxy resin 14; whereas, a decrease starts at 500xc2x0 C. in the case of the polyimide resin 12. In case of the PII resin 11, the heat resistance is more excellent, and the weight residue at 600xc2x0 C. is approximately 70% which is the most excellent.
Therefore, it is recognized that among those resin materials, only the PII resin and the polyimide resin can satisfy the aforementioned conditions (2) and (3), the other organic resin materials cannot satisfy the condition (3).
As heretofore described in the PII resin and the polyimide resin, the contents of uranium and thorium are extremely low. Accordingly, the PII resin and the polyimide resin stops the alpha-rays emitted from the ceramic package, and simultaneously, the alpha-rays to be generated by the alpha generators in the resin itself are in an extremely small amount. Therefore, these resins are very superior for the protective film to prevent soft error caused by the alpha rays. In order to avoid any fluctuation in the characteristics of the semiconductor memory device, the thickness of the PII resin film or the polyimide resin film should preferably be at least 10 xcexcm, more preferably at least 30 xcexcm. Both the PII resin and the polyimide resin can be said to be excellent materials also from the standpoint of the impurity contents of uranium, thorium, etc. As previously stated, however, the PII resin is more favorable due to its heat resistance.
Here, the xe2x80x9cpolyimide resinxe2x80x9d shall mean a polymeric material which is obtained by the reaction between aromatic diamine and aromatic tetracarboxylic acid dianhydride, while the xe2x80x9cPII resinxe2x80x9d shall mean a polymeric material which is obtained by the reaction among aromatic diamine, aromatic tetracarboxylic acid dianhydride and aromatic diaminocarboxamide. Both are well known, and the PII resin is described in detail along with a manufacturing method therefor in, for example, the official gazette of Japanese patent application publication No. 48-2956.
Since it is the semiconductor element that is influenced by the alpha-rays, a semiconductor substrate to be used in this invention includes at least one element or at least one active region which is affected by the entrance of the alpha-rays. Since that part of the semiconductor substrate which is affected by the alpha-rays is a portion of the element region, the coating film of the resin to be disposed for stopping the invasion of the alpha-rays must exist at least on the region of the element or the active region.
In the presence of an insulating layer, an electrode, an interconnection layer etc., the semiconductor substrate shall include them. The semiconductor devices of this invention are mainly constructed of monolithic integrated circuits.
The ceramic encapsulation is a technique well known in the field of semiconductor technology, and all the ceramic packagings having hitherto been employed can be applied. The ceramic packagings are, for example, those called xe2x80x9ccofired ceramic dipxe2x80x9d and xe2x80x9ccerdipxe2x80x9d. The ceramics usually contain aluminous ceramics as their principal constituents, but any of the ceramic materials having hitherto been employed may be used. Further, glass whose principal constituent is lead glass is employed for the bonding between the ceramics in the cerdip type. In the cofired ceramic dip type, on a ceramic package to which the semiconductor substrate is die-bonded, a cover made of a metal such as Kovar or a ceramic is bonded by the seam welding or with a binder such as eutectic alloy between Au and Sn.
The polyimide resin and the PII resin (especially the polyimide resin) sometimes contain some (on the order of several ppm) alkali impurities such as Na. In this case, when the semiconductor substrate is formed thereon with the resin film of, for example, the polyimide resin and is subjected to a heat treatment at a high temperature, the alkali impurities permeate into the interior of the semiconductor substrate through pinholes etc. existent in an insulating film disposed on the surface of a semiconductor sheet constituting the semiconductor substrate, and they can change the characteristics of the element. To the end of preventing this drawback, it is effective that a phosphosilicate glass film or a silicon nitride film which has a high capability of checking the permeation of the alkali ions is formed on the semiconductor substrate and interposed between it and the polyimide resin.
The foregoing semiconductor device of this invention can be readily manufactured by a producing method including: (i) the step of covering at least a region of an element in a semiconductor substrate having at least one element, with a polyimide resin or a PII resin having a thickness of 10 xcexcm or more, and (ii) the step of encapsulating in a ceramic package the semiconductor substrate covered with the resin.